Don't know what you mean by the Torturer vs Mandarin example, I don't get what comparing one of the worst $5 cards with one of the best $5 cards (and obviously this has little to do with the cards vs coin, but the secondary effects) is supposed to be saying.
I don't mean to sound harsh, but most of the arguments you bring up are either statements that mean nothing by themselves ($2 may or may not be the strongest combination of "2 vanilla bonuses", but then again there's a reason no other cards in the game are just "2 vanilla bonuses") or just misinformed. Saying "Herbalist is much worse except for the two good things about it" and "Moat is much worse except for the primary effect of it" is just pointless. Yes, remove +buy and the ability to reuse Gold/Platinum/Hoard/whatever and Herbalist is obviously a lot worse than Duchess, but what does that even mean? Yes, Moat is often (maybe even "almost always") a mistake in 2p games even with attacks, but Duchess is probably a bigger mistake, more often. Again, pointing out that Moat (one of the weaker cards in the game) is horrible doesn't make Duchess less horrible. What makes Duchess worse than other bad $2 cards is that there are (very very close to) no situations where you want it (talking primarily about the early/mid game, see above post for comment on endgames). Moat and Herbalist, you rarely want, but when you do, they actually provide something of value.
Pearl Diver is sort of a pet hate card of mine, so we're kinda in agreement I guess. That said, a card that (given it's in your deck already) 2% of the time is helpful, 5-20% of the time is harmful, and 78-93% of the time is pointless, is much much better than a card which is harmful 20-50% (edit: this is kind of a stupid way of evaluating things I'll admit, since cards don't magically pop into a deck without consideration, so don't be too hard on me for using those numbers) of the time (given it's in your deck).
On the vast majority of boards, you prefer opening $5 card/nothing with a 5/2 split rather than $5 card/Duchess. The same is true with Moat, Pearl Diver, Herbalist, and more cards, but it's true less often of those cards, and those cards are much less bad (or "better" sometimes, even), than Duchess (basically) ever is.